fix: clarify PGRST116 error message (#3795)

Currently it's redundant and not easy to read.

```
{"message":"JSON object requested, multiple (or no) rows returned",
"details":"The result contains 2 rows"}
```

Now:

```
{"message":"Cannot coerce the result to a single JSON object",
"details":"The result contains an array of 0 objects"}
``

Also correct docs which had a wrong error code.

src/PostgREST/Error.hs

@@ -163,7 +163,7 @@ instance JSON.ToJSON ApiRequestError where
 
   toJSON (SingularityError n) = toJsonPgrstError
     ApiRequestErrorCode16
-    "JSON object requested, multiple (or no) rows returned"
+    "Cannot coerce the result to a single JSON object"
     (Just $ JSON.String $ T.unwords ["The result contains", show n, "rows"])
     Nothing